Rotor assembly for a rotorcraft

ABSTRACT

A rotor assembly ( 5 ) for a rotorcraft, such as a helicopter, includes rotors ( 7 ) rotatable about a hub ( 17 ). The assembly ( 5 ) is operable to vary the angular speed of the rotors ( 7 ) about the hub ( 17 ), relative to one another. The assembly ( 5 ) may include a drive operable to drive each of the rotors ( 7 ) at a different radial distance from the hub.

TECHNICAL FIELD

The present invention relates to rotorcraft, and more particularly torotor assemblies for rotorcraft.

BACKGROUND OF THE INVENTION

During forward motion of a rotorcraft, such as a helicopter, there is adifference between the relative air speed of an advancing rotor and aretreating rotor. As the lift generated by a rotor is dependent on thespeed the rotor passes through the air, this imbalance in relative airspeed tends to shift the centre of lift to one side of the rotorcraft.To maintain a balanced lift profile across the rotorcraft, the higherlift generated by the faster-moving advancing rotor is typically offsetby decreasing its pitch (i.e. its angle of attack) relative to theretreating rotor. Nevertheless, the maximum forward speed of arotorcraft may still be limited because the retreating blade mayapproach stall and/or the tip region of the advancing blade may approacha supersonic speed.

The pitch control on known rotor assemblies may also enable therotorcraft to be manoeuvred by moving the centre of lift away from theaxis of rotation. Known pitch control mechanisms can give rise to anumber of disadvantages. For example: the mechanisms tend to beextremely complex and typically comprise various hinges and levers whichhave high maintenance costs and onerous safety requirements; excessivenoise and vibration can occur because the pitch of the blade is altered,at relatively high frequency, during revolution of the rotors; and/orthe rotor hub must be extremely strong as it is subjected to theintertial loads of the rotors and the loads associated with pitching therotors, and it also transmits the lift force from the blades to therotorcraft fuselage.

SUMMARY OF THE INVENTION

The present invention seeks to remove or mitigate at least some of theabove-mentioned disadvantages.

According to a first aspect of the invention, there is provided a rotorassembly for a rotorcraft, the assembly comprising a plurality of rotorsrotatable about a hub, wherein the assembly is operable to vary theangular speed of the rotors about the hub, relative to one another.Varying the relative angular speed of the rotors around the hub changesthe lift distribution around the hub and shifts the centre of lifttowards the faster moving blades. The lift imbalance caused by thediffering relative air speed of the advancing and retreating blades inforward flight, can therefore be offset. This may allow a higher maximumforward speed to be attained. Varying the relative angular speed of therotors (and therefore the lift distribution around the hub) may also beused to manoeuvre the rotorcraft.

When there is a difference in the relative angular speed of the rotors,the angular spacing between the rotors will vary accordingly. A changein lift may also be generated by this angular imbalance in the spacingof rotors around the hub. As the lift of the rotors is approximatelyproportional to the square of their speed, the increase in lift as aresult of their speed increase tends to more than offset any decreaseincrease in lift caused by a reduced lifting surface (the rotors willtend to be further apart when travelling faster and closer together whentravelling slower).

The angular speed of the rotors around the hub is preferably arranged tovary in a cyclic manner. The angular speed of the rotors may be arrangedto vary in a sinusoidal manner. A sinusoidal variation has been found tofacilitate a particularly smooth acceleration/deceleration of the rotorsaround the hub. The rotors may be independently controllable but arepreferably all arranged to follow the same cyclic variation in angularspeed around the hub. The cycle is preferably repeated once everyrevolution of the rotors around the hub.

The rotors are preferably arranged in substantially the same plane ofrotation.

Known rotorcraft rotor assemblies tend to comprise a plurality of rotorsthat are fixedly mounted to the circumference of a rotatable hub. Incontrast, according to the first aspect of the invention, the assemblycomprises a plurality of rotors rotatable about a hub. Preferably thehub is fixed, for example it is fixedly mounted on the rotorcraft.

It is desirable to be able to rapidly accelerate the rotors in a rotorassembly as this may enable a quick take-off. A rapid acceleration ofthe rotors may also reduce the need for the rotors to be rotated up toan idling speed long before take off (for example when crew and/orpassengers are embarking), thereby improving safety. It can also bedesirable to be able to quickly decelerate the rotors in a rotorassembly, for example so that the rotors can be stationary duringdisembarkation of the crew/passengers.

Rotor assemblies on known rotorcraft typically comprise a rotating hubhaving a plurality of rotors fixed thereto. The rotor assemblies tend toinclude a complex hinge mechanism for varying the pitch of rotors as thehub rotates. There is a limit to how quickly these known rotorassemblies can change their rotational speed. In particular, the maximumtorque such arrangements can withstand tends to be limited by the needto keep the weight and complexity of the hinge mechanism within certainpractical limits.

The assembly may comprise drive means arranged to drive the rotors at aradial distance away from the hub. Driving the rotors around a hub at aradial distance from that hub has been found to give rise to a number ofbenefits. For example, the assembly may be able to withstand a greatertorque than some known assemblies and may therefore be operable to morequickly accelerate or decelerate the rotor blades. Rapid acceleration ofthe rotors may also be used in combination with pitch variation of therotors to quickly manoeuvre the rotorcraft.

The drive means is preferably arranged to drive the rotors by applying adrive force at the radial distance from the hub. The drive means arepreferably arranged to drive the rotor at a radial distance from thesurface of the hub. The drive means are preferably arranged to drive therotor at least 5%, and more preferably 10%, and yet more preferably 15%of the length of the rotor. The drive means may be arranged to drive therotor at least 0.2 m, and more preferably at least 0.5 m along thelength of the rotor.

The assembly preferably comprises a clutch for engaging and disengagingrotation of the drive means from a power source for powering the drivemeans (for example an engine).

The power source for driving the drive means may therefore be switchedon (for example to warm up and/or allow safety checks to be performed)without rotating the rotors. This enables certain advantages, forexample, passengers or crew can be loaded whilst the engine is runningwithout being subjected to the noise and vibration of the rotatingblades. In embodiments of the invention in which the rotor assembly isarranged to withstand relatively high torque, the power source may beable to be run to a relatively fast operating speed before engaging withthe drive means, thereby rapidly accelerating the rotors.

The drive means may be operable to drive each of the plurality of rotorsat a different radial distance from the hub. Driving the rotors atdifferent radial distances may facilitate a relatively simplearrangement for varying the angular speed of the rotors. For example, ifthe rotors are driven about different radii at the same tangentialspeed, the relative angular speed between the blades will be varied.

The drive means may comprise a rotatably driven drive ring, the rotorsbeing coupled to the circumference of the drive ring. The drive ring ispreferably circular. The drive ring may be in the form of a disc, but ispreferably in the form of an annulus. The rotors are preferably coupledto the circumference of the drive ring such that the circumferentialspacing between the rotors around the circumference of the drive ring isfixed. It will be appreciated that due to the thickness of the rotors,there may be a small change in the circumferential spacing of the rotorsif the angular spacing of the rotors changes during rotation of thedrive ring. It will be appreciated however, that the circumferentialspacing between at least part of each rotor (for example the centralaxis of the rotor that typically passes along the length of the shaftand through the centre of lift of the rotor) is fixed.

The rotor assembly is preferably arranged to be operable between afirst, neutral lift, configuration in which the axis of rotation of therotors is co-axial with the axis of the hub, and a second, offset lift,configuration in which the axis of rotation of the rotors is spacedapart from the axis of the hub. The axes of rotation may be parallel inboth the first and second configurations.

The drive ring may be moveable, preferably laterally moveable, relativeto the hub, such that the drive ring is operable to drive each of theplurality of rotors at a different radial distance from the hub. Thedrive ring is preferably laterally moveable to move the axis of rotationof the drive ring from a configuration in which it is co-axial with thehub, to a configuration in which it is spaced apart from the axis of thehub. The assembly may comprise an actuator for moving the drive ring.The assembly may comprise a plurality of actuators (for example a pairof orthogonally positioned actuators) for moving the drive ringforward/back, left/right, or a combination of the two. The drive ringmay be rotatable around a track. The track may be laterally moveable. Itwill be appreciated that a lateral movement may be any movement in anydirection within the horizontal plane containing the drive ring (forexample forward/aft, left/right, or a combination thereof). The movementis preferably transverse to the axis of the drive means.

The drive means may be coupled to a drive shaft via a belt arrangement.In embodiments in which the drive ring is rotatable around a track, thebelt may be arranged to rotate the drive ring in the track.

The assembly may be arranged such that the tangential speed of therotors at each radial distance is equal. For example, by having therotors coupled to the circumference of the drive ring such that thecircumferential spacing between the rotors around the circumference ofthe drive ring is fixed, the tangential speed of each rotor at thecircumference of the drive ring, is equal. In an embodiment in which therotors are being driven around the fixed hub at different radialdistances around the hub, the relative angular speed of the rotors willvary accordingly around the hub.

The rotors may be slideably moveable through the circumference of thedrive ring in a direction along the length of the rotor, such thatlateral movement of the drive ring alters the angular spacing of therotors. This arrangement allows the driving force from the drive meansto be applied at different radial distances along the rotor. The drivering may comprise a plurality sleeves through which the rotors pass. Thelocation of the sleeves on the circumference of the drive ring ispreferably fixed, but the sleeves may be pivotably mounted to enable theangle of the sleeve to vary during rotation of the drive ring.

Each rotor may be mounted on the hub via a mounting, the drive meansbeing arranged to drive the rotors at location radially outward of themounting. The mounting may be slideably moveable around thecircumference of the hub. The hub may comprise a track in which themounting is slideably received. The assembly may be arranged such thatthe mountings of adjacent rotors may, at least partially, overlap aroundthe circumference of the hub. For example, the hub may comprise twocircumferential tracks; an outer track and an inner track located withinthe outer track. Adjacent mountings may be received in inner and outtracks respectively. The mountings may be arranged such that themounting on the inner track is receivable, at least partially, withinthe mounting on the outer track. The adjacent mountings are preferablydifferently configured.

Some known rotor assemblies in which the rotors are fixed to a rotatablehub, and limited to having relatively few rotors due to the complexityof the hub mechanism. Embodiments of the present invention may provide asimplified arrangement and therefore enable more rotors to be mounted onthe rotor assembly. The assembly may comprise at least 4 rotors, andmore preferably at least 6 rotors. The assembly may comprise at least10, or even 12 rotors. The assembly may comprise up to 20 rotors. Havinga relatively large number of rotors may reduce imbalances and/orvibrations that arise from shifting the relative spacing between theblades.

The more rotors, the higher the lift for a given rotational speed.Accordingly, to achieve a given lift with an assembly comprising arelatively large number of rotors, the rotational speed may be reduced.By running the rotor assembly at a lower speed the vibration and sound(which increase logarithmically or in a squared relationship with speed)may be reduced. The impact of forward motion on the relative speeddifference between the advancing and retreating rotors is proportionallymore significant at lower rpm. Arrangements in which the relativeangular speed of the rotors may be varied are therefore particularlyadvantageous when used in conjunction with rotor assemblies operating atlow rpm.

Alternatively or additionally, other parameters of the rotors (such asthe length of the rotors, their area or their lift co-efficient) may bechanged to offset the increase in lift caused by a higher number ofrotors.

Preferably the rotor assembly comprises load transferring assembly,located radially outward of the junction between the rotor and the hub,the load transferring assembly being arranged to transfer at least someof the lifting force generated by the rotors, to a location away fromthe hub. The load transferring assembly may therefore reduce certainloads on the hub. In embodiments of the invention comprising a housing,the lifting force is preferably transferred to the housing.

The variation of pitch of a rotor blade is achieved in many knownhelicopter configurations by the use of a swash-plate mechanism; acircular planar surface centered perpendicular to the main drive shaftthat can be tilted and in some cases moved vertically with respect tothe central drive shaft about which the rotor blades rotate. The pitchof each rotor blade is typically governed by the height of the surfaceof the swash plate at the corresponding position about the circumferenceof the central drive shaft. A further aspect of the invention providesan alternative method to vary the rotor pitch. According to this furtheraspect of the invention there is provided a rotor assembly comprising aplurality of rotors and a pitch controller for varying the pitch of therotors, wherein the pitch controller comprises a guide means and alinkage moveable along the guide means, the linkage being connected tothe rotor such that the pitch of the rotor is variable in dependence onthe separation between the rotor and the guide means. The pitchcontroller may be arranged on the rotor assembly according to the first,or any other, aspect of the invention. Each rotor may comprise a bladehaving a tip and a root, and a shaft extending from the root of theblade to the hub. The shaft may be coupled to the blade in a variety ofways; for example the blade may be fixedly coupled to the shaft, or theblade may be rotatable relative to, and for example about thelongitudinal axis of, the shaft. The linkage may be connected to theblade of the rotor.

The pitch controller may comprise an actuator arranged to vary theseparation between the rotor and guide means. The pitch controller maycomprise a plurality of independently actuatable actuators arranged tovary the separation between the rotor and guide means. The actuatorsmay, for example, be hydraulic actuators. The actuators may be arrangedto vary the height of the guide means relative to the rotors.

Known swashplate assemblies typically comprise a rigid, planar, platethat is tilted at an angle to the axis of rotation of the rotors. In anembodiment of the present invention, the guide means may be flexiblesuch that the guide means may be deformed from a planar configuration toa non-planar configuration. By deforming the guide means to a non-planarconfiguration, the pitch controller may provide a greater variation inpitch during one revolution of the rotors. This is particularly ofbenefit in an arrangement in which the angular speed of the rotors canbe varied, because it can be used in conjunction with the speedvariation to control the lift distribution. Adjustments may be made thatsupport complex variations of the pitch about the circumference. Thismay facilitate more refined maneuverability, counteract vibration andmight improve or resolve other behaviors.

The guide means may comprise a ring member. The ring member may comprisea circumferential track along which the linkage is moveable. The linkagemay be arranged in relation to the guide means such that both tensileand compressive loads in the linkage are reacted through the pitchcontroller. For example, the track may be in the form of a groove alongthe outer edge of the ring member and an end of the linkage may bereceived in the groove, the tensile and compressive loads being reactedagainst the upper and lower surfaces of the groove respectively.

The guide means may be located at substantially the same radial distancefrom the hub as the root of each rotor. The guide means may be locatedat a radial distance of at least 0.5 m, and more preferably at least 1m, from the centre of the hub. The guide means is located at a radialdistance of at least 10% and more preferably at least 20% of the lengthof the rotor, from the centre of the hub. By moving the pitch controlaway from the hub, the loads on the hub may be reduced. The loadsgenerated by varying the pitch of the blade are preferably de-coupledcompletely from the hub. The mechanical complexity of the hub may besimplified thereby facilitating different configurations of the rotorassembly.

The guide means may be fixed such that the linkage rotates with therotors relative to the guide means. Having the guide means fixed and thelinkage rotating in the guide means, may enable the pitch controller tobe simplified. The guide means is preferably de-coupled from the hub.The guide means may be arranged to react loads through the actuators ata radial distance away from the hub.

In an embodiment in which the rotors are driven at a radial distanceaway from the hub, the guide means may be located radially outward ofthe radial distance at which the drive means is arranged to drive therotors.

As well as varying the pitch of the rotors, or alternatively thereto,the rotor assembly may be arranged to vary the flap angle of the rotors.This provides another way of varying the lift distribution of the rotorassembly and therefore also facilitates better control andmanoeuvrability using the rotor assembly. The assembly may comprise aflap controller, the flap controller comprising a second guide means anda second linkage moveable along the guide means, the linkage beingconnected to the rotor such that the flap angle of the rotor blade isvariable in dependence on the separation between the rotor and thesecond guide means. The flap controller may comprise any of the featuresdescribed above with reference to the pitch controller. It will beappreciated that the presence of the second guide and the second linkagedoes not necessarily mean that the first guide and first linkage of thepitch control are essential. There may be embodiments of the inventionin which there is no pitch controller, and only a flap controller. Theflap angle of the blade may be varied by simply bending the rotor, butmore preferably the rotor comprises a hinge about which the flap angleof the rotor blade may vary.

The rotor assembly preferably comprises both a pitch controller and aflap controller. In such an embodiment the pitch controller ispreferably mounted on the flap controller. For example an actuator ofthe pitch controller may be mounted on the second guide means of theflap controller. Such an arrangement allows the pitch and flapcontroller to be stacked above one another and may therefore provide arelatively compact arrangement. Mounting the pitch and flap controllerson one another may avoid the need to have the controllers on oppositesides of the plane of rotation of the rotors (which would requireespecially strong mountings both below and above the rotors).

According to another aspect of the invention there is provided a rotorassembly comprising a plurality of rotors, each rotor comprising a bladefor generating lift, the blade having a tip and a root, and a shaftextending from the root of the blade to a hub, wherein the length of theshaft is at least 10%, and more preferably 20%, of the length of therotor, such that the innermost region of the rotor generatessubstantially no lift. The length of the shaft may be more than 25% ofthe length of the rotor. The shaft may be at least 0.5 m and preferablyat least 1 m long. The inner region of a rotor blade generatesrelatively little lift in comparison to the outer regions of a rotor(which travel much faster). Moving the rotor blade further from the hubhas been found to produce a relatively small decrease in lift with theadvantage that the downdraft close to the hub (which is typically in theregion above the rotorcraft cabin) is significantly reduced. This, inturn, has been found to reduce the noise and vibration caused by therotor assembly.

The above-described rotor arrangement may be incorporated in the rotorassembly according to the first, or any other aspect of the invention.

The assembly may comprise a housing. The housing may have an upper coverportion extending over the shafts of the rotors, the housing being fixedsuch that the rotors rotate relative to the housing. The housing may bea disc structure located above the rotor shafts. The radius of the discstructure is preferably greater than or equal to the radius at which theshaft meets the root of the rotor blade. Thus, the housing preferablycovers the shafts when view directly from above.

The housing may substantially enclose the shafts of the rotors. Thehousing preferably comprises a barrier. The rotors preferably passthrough the barrier. The assembly is preferably arranged such that thebarrier substantially isolates the inside of the housing from theenvironment outside the housing. Such an arrangement protects thecomplex moving parts inside the rotor assembly from the environmentoutside the assembly, thereby reducing the need for maintenance andcleaning. The rotor assembly may comprise a means for maintaining apositive pressure inside the housing, relative to outside the housing.

According to a further aspect of the invention there is provided a rotorassembly comprising a plurality of rotors, a hub and a housing, eachrotor comprising a blade for generating lift, the blade having a tip anda root, and a shaft extending from the root of the blade to the hub, thehousing substantially enclosing the shafts of the rotors and furthercomprising a barrier such that the inside of the housing issubstantially isolated from the environment outside the housing. Thebarrier may comprise a plurality of plates, slideably mounted relativeto one another. The housing is preferably on the rotor assembly asdescribed herein with reference to any aspect of the invention.

According to another aspect of the invention, there is provided arotorcraft including the rotor assembly according to any aspectdescribed herein. The rotorcraft is preferably a helicopter. Therotorcraft is preferably at least 10 kg dry weight, more preferably atleast 50 kg dry weight, and yet more preferably considerably more than50 kg dry weight. The rotorcraft is preferably suitable for carrying atleast 1 person, and more preferably at least 2 persons, and yet morepreferably at least 4 persons. The rotorcraft may be suitable forcarrying more than 10 persons.

According to yet another aspect of the invention, there is provided amethod of moving the centre of lift of a rotorcraft, the methodcomprising the steps of rotating a plurality of rotors about a hub suchthat the angular speed of the rotors about the hub, relative to oneanother, is varied.

It will be appreciated that any features described with reference to oneaspect of the invention may be equally applicable to another aspect ofthe invention. For example, features describe with reference to anyaspect of the rotor assembly may be equally applicable to the method ofthe invention.

DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings ofwhich:

FIG. 1 is schematic side view of a helicopter according to a firstembodiment of the invention;

FIG. 2 is a schematic plan view of the helicopter of FIG. 1;

FIG. 3 is a plan view of the rotor assembly on the helicopter of thefirst embodiment of the invention;

FIG. 4 is a section view through the line A-A in FIG. 3;

FIG. 5 is a partial plan view of the rotor assembly of FIG. 3,

FIGS. 6 a and 6 b are sectional view of the rotor mounting trolleys inthe rotor assembly of FIG. 3;

FIG. 7 is a schematic plan view of the rotor assembly of FIG. 3 showingthe drive mechanism for driving the drive ring;

FIG. 8 is a plan view of the rotor assembly of FIG. 3 with the drivering laterally shifted to one side;

FIG. 9 is a partial plan view of the drive ring in the rotor assembly ofFIG. 3;

FIG. 10 is a schematic plan view of the rotor assembly of Figure showingthe variable spacing of the rotors;

FIG. 11 is a sectional view of part of a rotor assembly according to asecond embodiment of the invention;

FIG. 12 is a perspective view of the rotor assembly of FIG. 11;

FIG. 13 is a sectional view of part of a rotor assembly according to athird embodiment of the invention;

FIG. 14 is a view of part of the rotor assembly of FIG. 13; and

FIGS. 15 a and 15 b are views of the part of the rotor assembly in twodifferent configurations.

DETAILED DESCRIPTION

According to a first embodiment of the invention, a helicopter 1comprises a fuselage 3 and a rotor assembly 5 mounted on the top of thefuselage 3. The rotor assembly carries twelve rotors 7, each rotorcomprising a 3 m long blade 9 having an aerofoil section (the fulllength of only the forward and aft rotors is shown in FIG. 2, only theinner region of the other rotors being shown). The blade 9 extends froma root 11 to a tip 13. For the sake of clarity, each rotor blade 9 isshown having a uniform chord and zero twist/washout. In practice, thechord and twist of the blade 9 vary along the length of the blade as iswell known in the art. A 1 m long shaft 15, aligned with the centre oflift of the blade 9, extends from the root 11 of the blade to a hub 17.The rotor assembly 5 comprises a disc-shaped cover portion 19 which isfixedly attached to the top of the hub 17. The cover portion extendsoutwardly to the root 11 of the rotor blades 7. A dome-shaped cowling 21is attached to the top of the cover portion 19.

The rotor assembly 5 is shown in more detail in FIG. 3 (a plan view ofthe rotor assembly with the cover portion 19 removed) and FIG. 4 (asectional view along the line A-A of FIG. 3). Referring to FIGS. 3 and4, the twelve rotors 7 are rotatable about the central hub 17. The hub17 is fixedly mounted on the helicopter fuselage 3.

The cover portion 19 extending out from the top of the hub 17 has a lip23 extending downwardly from its circumference and the lip 23 has asemi-circular groove 25 in its underside. A free-coasting wheel 27 ispositioned on the shaft of each rotor close to the junction of the shaft15 and blade root 11. The wheel 27 is arranged to run in the groove 25.In this manner much of the lift force from each rotor 7 is transmittedto the cover portion 19 and hence to the helicopter fuselage 3.

The free end of each rotor shaft 15 is held in a mounting trolley 29which is, in turn, received in a circumferential track 31 extendinground the hub. Each trolley 29, and hence each rotor 7, is freelyrotatable around the fixed hub 17. This is in contrast to conventionalrotorcraft rotor assemblies, where the rotors are typically fixed to thehub and the hub is rotatably driven.

In order to allow a plurality of rotors to rotate about the same fixedhub, various different mechanisms can be employed. The hub 17 and track31 arrangement of the first embodiment of the invention will now bedescribed in more detail: Referring to FIG. 5 and FIGS. 6 a and 6 b, thehub 17 comprises a circumferential track 31 with an opening 33 throughwhich the rotor shaft 15 extends. Radially innermost in the track 31,there is an outer pair of circumferentially extending notches 35 a andan inner pair of circumferentially extending notches 35 b. Radiallyoutermost in the track 31 there are a single pair of semi-circularcircumferential grooves 37.

The assembly comprises two types of mounting trolley 29 a and 29 b. Eachtype of mounting trolley 29 a, 29 b is positioned adjacent to one of theother type. Each trolley comprises a set of wheels 39 received in thepair of semi-circular circumferential grooves 37, but the differenttypes of trolley have wheels in the outer 35 a and inner 35 b pairs ofnotches respectively. Thus the trolley 29 a for the outer notches 35 ahas a generally wedge-shaped support structure whereas the trolley 29 bfor the inner notches 35 b has a generally box-shaped support structure.The box-shaped support structure can be at least partially received inthe wedge-shaped support structure and thus the adjacent mountingtrolleys 29 a and 29 b are operable to overlap around the circumferenceof the hub 15 (the benefits of which are discussed later). In addition,the arrangement in the first embodiment of the invention can sustainsignificant rotational and lift forces.

As discussed above, the rotors 7 are freely rotatable around the fixedhub 17. In the first embodiment of the invention, the rotors 7 aredriven by applying a driving force at a radial distance along the shaft15, close to the root 11 of the blade 9. To drive the rotors 7 in thisfashion, the assembly 5 comprises a drive ring 39. Referring back to

FIGS. 3 and 4, the drive ring 39 is received in, and is freely rotatablein, a circular track 41 fixed to the top of the helicopter fuselage 3.The mechanism for driving the drive ring 39 is not shown in FIGS. 3 and4 for the sake of clarity. Instead, FIG. 7 shows the drive mechanism byway of a schematic plan view of the rotor assembly 5. The drive ring 39is rotated by way of a belt 43 engaging a groove 45 in the outer edge ofthe drive ring 39, the belt 43 also passing round a roller 47, which isin turn driven by a second belt 49 engaged with the output shaft 51 ofan engine 53. By driving the rotors by the drive ring, thereby applyinga drive force at a radial distance from the hub, the rotor assembly isable to withstand a relatively large torque (in comparison to anarrangement in which the rotors are fixed to a rotating hub). Thisenables a large torque to be safely applied and enables the rotors to berapidly accelerated.

This belt drive arrangement enables the drive ring 39 to be rotated evenwhen it is laterally repositioned. FIG. 7 shows the drive ring laterallyrepositioned to the right and lateral movement of the ring is indicatedby the double-headed arrow. The mechanism for laterally moving the drivering, and its effects, are described below.

The circular track 41 in which the drive ring 39 is received is moveablelaterally by way of a hydraulic actuators 55 fixedly mounted on the hub(the actuator is not shown in FIG. 3 for clarity). At maximum extension,the actuator 55 positions the track 41, and hence the drive ring 39,over the far right-hand side of the assembly. FIG. 8 is a schematic planview of the rotor assembly showing the drive ring 39 at close to maximumdisplacement to the right-hand side.

The structure of part of the drive ring is shown in more detail in FIG.9. The drive ring 39 comprises a ring-shaped base portion 57 on whichtwelve sleeves 59 are pivotably mounted. For the sake of clarity, FIG. 9only shows one half of the drive ring and only one of the sleeves. Eachsleeve 59 comprises hollow tube portion 61. The tube portion 61 isconnected to a protruding flange 63 through which a vertical spindle 65passes. The spindle 65 defines an axis that is parallel to the axis ofrotation of the drive ring 39. The shaft 15 of a rotor 7 passes througheach respective sleeve 59 and is slideably received therein. Relativemovement between the rotor 7 and drive ring 39 along the length of eachrotor is therefore enabled, but circumferential movement between therotor 7 and drive ring 39 at the point the rotor 7 passes through, andis coupled to, the circumference drive ring 39, is prevented.

FIG. 10 is a highly schematic plan view of the rotor assembly showinghow the rotors are arranged when the drive ring has been laterallyshifted to the right. As explained above with reference to FIG. 9, thecircumferential spacing (c) of the rotors around the drive ring isfixed, but the rotors are arranged such that they are slideably moveablealong their length relative to the drive ring. This means the angularspacing (a1, a2 and a3) is varied when the drive ring is movedlaterally.

When the drive ring 39 and the hub 17 are co-axial, the shafts 15 of therotors 7 all pass through the sleeves 59 on the drive ring 39 at thesame radius. In this case, when the drive ring 39 is driven at aconstant angular speed, all the rotors 7 rotate at this same angularspeed. When the drive ring is laterally repositioned (for example to theright of the hub as shown in FIG. 10), the angular speed of each pointon the drive ring is still constant, but that angular speed occurs at adifferent radius r1, r2 and r3 (from the hub) along each rotor. In theexample shown in FIG. 10, the radius at which each rotor is being drivenis greater on the right-hand side than the left-hand side. Therotational speed of each rotor therefore varies throughout onerevolution of the blades. In the schematic diagram shown in FIG. 10 theradius of the drive ring is 0.75 m and the offset from the axis of thehub is 0.25 m. The drive ring is rotating at an angular speed of Ω. Thetangential local speed of the left-hand rotor at the point at which itpasses through the drive ring must be equal to the tangential localspeed of the right-hand rotor at the point at which it passes throughthe drive ring as circumferential spacing of the rotors at these pointsis fixed. In the example in FIG. 10, both those speeds are equal to0.75*Ω. However, the rotors are rotating around the hub, and aretherefore being driven at different radii. The angular speed of theleft-hand rotor is therefore 0.75*Ω/(0.75−0.25)=1.5Ω, whereas theangular speed of the right-hand rotor is 0.75*Ω/(0.75+0.25)=0.75Ω. Theangular speed of each rotor is therefore different. In theabove-mentioned example, the rotor is driven at 430 rpm, and the rotorassembly is arranged to compensate for a forward speed of around 150 mphwithout needing to resort to pitch control. In the first embodiment ofthe invention, the angular speed of the rotors varies sinusoidallywithin each revolution of the drive ring. The maximum speed differentialbetween the rotors occurs between rotors on opposing sides of the hub ina direction that is parallel to the direction of the lateral shift ofthe drive ring.

By laterally moving the drive ring to one side, the retreating rotorscan be rotated faster than the advancing rotors. This can offset anydifference in relative airspeed caused by forward motion of therotorcraft, and therefore increases the maximum forward speed of therotorcraft and reduces the need for pitch control of theadvancing/retreating blades.

When the drive ring is moved laterally, the rotor assembly is subjectedto significant inertial loads. To reduce rotational speed at which therotor assembly operated (and therefore the inertial loads), the rotorassembly is provided with a relative high number of rotors (twelve inthe first embodiment of the invention). Furthermore the rotors arerelatively short (approximately 4 m, with a 3 m blade) and light.

Although the ability to vary the relative speed of the rotors may reducethe need to have a pitch control, in a preferred embodiment the rotorassembly still comprises a pitch control mechanism. Referring back toFIGS. 3 and 4, the rotor assembly 5 comprises a pitch control guide ring67 mounted on four equally spaced hydraulic actuators 69. The ring 67 isconcentric with the hub 17, and when all the actuators 69 are at thesame height the ring 67 is level (relative to the fuselage 3 of therotorcraft 1) and co-axial with the hub 17. The actuators 69 areindependently controllable to change the total elevation and/or theangle of the guide ring 67. The pitch control guide ring 67 comprises acircumferential channel 71 passing around the outer edge of the ring 67.

A pitch control linkage 73 is connected, at one end, to the trailingedge of the rotor blades 9. The other end of the linkage 73 includes arotatable wheel 75 located in the channel 71 passing around thecircumference of the guide ring 67. As the rotors rotate, the wheel 75on the end of the linkage 73 is pulled around the channel 71. When theguide ring 67 is horizontal relative to the hub (i.e. is generally inthe plane of rotation of the rotors), the pitch of all the rotor blades9 is equal throughout one revolution of the rotor assembly. However,when the angle of the guide ring 67 is altered, the pitch controllinkage 73 pushes or pulls the trailing edge of the blade up or down asit passes round the guide ring 67 thereby altering the pitch (i.e. angleof attack) of each blade 9 as it passes through one revolution.

In the first embodiment of the invention, the pitch control linkage 73is located around 1 m from the axis of rotation of the rotors. This isin contrast to known rotor assemblies where the pitch control mechanismtends to be located on, or very close to, the hub of the rotor assembly.By positioning the pitch control away from the hub, the mechanicalcomplexity of the hub is reduced. The loads to which the hub issubjected are also reduced because at least some component of the liftmay be transmitted to the helicopter fuselage via the guide ring 67(located well away from the hub) and the hydraulic actuators 69 to whichit is attached.

The helicopter of the first embodiment of the invention has twelverotors, which is more than a typical helicopter (which tend to havebetween 2 and 4 rotors). Due to the greater number of rotors, the bladelength can be reduced relative to a typical helicopter, withoutdecreasing the lift at a given rotational speed. As discussed above,each rotor comprises a relatively long shaft 15 extending from the hubto the root 11 of the rotor blade 9. This shaft 15 is of circular crosssection and does not generate a significant, if any, amount of lift. Thedowndraft in the region of the hub 17 is therefore reduced relative toarrangements in which the rotor blade extends close to the hub. This hasbeen found to reduce the operating noise of the rotor assembly in thecabin of the helicopter which is positioned below the rotor assembly.

FIGS. 11 and 12 show a second embodiment of the invention. The secondembodiment of the invention is generally the same as the firstembodiment except for the differences described below. The equivalentfeatures are numbered with the same reference numerals except for aprefix of 1, or 10 as appropriate. According to the second embodiment,the rotor assembly comprises a flap controller and a pitch controller.The pitch controller is arranged in similar manner to that in the firstembodiment of the invention, except that the pitch guide ring 167 islocated above the blade and the actuators 169 are not attached to thefuselage. Instead, the actuators 169 are mounted on a second guide ring177 which is part of a flap controller.

The flap controller comprises the flap guide ring 177 attached to theflap actuators 179. A linkage 181 links the flap guide ring 177 to theshaft of each rotor at a position close to the root 111 of the blade109, and radially outward of a flap hinge 183. The linkage 181 ismoveable around the circumference of the guide ring 179 as the rotorsrotate about the hub. To change the flap angle of the rotor blades theactuators 179 move the guide ring 177 up or down as appropriate. Thischanges the separation between the guide ring 177 and the shaft 115,causing the rotor to rotate about the hinge 183.

The rotor assembly of the second embodiment of the invention comprisesonly four rotors 107 rotatable about the fixed hub 117. FIG. 12 is aperspective view of the rotor assembly showing an arrangement in whichthe flap angle on one side of the rotor assembly is increased. Byincreasing the flap angle, a lateral thrust is generated over this sideof the rotor assembly. The flap controller may therefore be used tomanoeuvre the rotorcraft. Of course, the second embodiment of theinvention also allows independent or complimentary pitch control via thepitch controller. If it is desirable to keep the pitch constant, whilstvarying the flap angle, the flap actuator 179 and the pitch actuator 169are controlled such that there is no net movement of the pitch linkage173.

FIG. 13 is a close up view of part of a rotor assembly according to athird embodiment of the invention. The rotor assembly is generally thesame as the first embodiment except for the differences described below.The equivalent features are numbered with the same reference numeralsexcept for a prefix of 2, or 20 as appropriate.

Upper and lower support surfaces 289 a and 289 b are located at theouter circumference of the rotor assembly housing. The support surfaceshave opposing grooves for receiving the upper and lower lengths of acircumferential plate 291. The plate 291 is made of flexible plastic andis constrained within the grooves such that the plate curves around withthe circumference of the housing.

FIG. 14 shows a side view of the plate (the plate is shown as being flatfor the sake of clarity). The plate 291 includes a hole 293 throughwhich the shaft 215 of the rotor passes. A curved slot 295 is alsocut-out below the hole 293 to accommodate the pitch linkage 273. Eachend of the plate comprises further cut-outs 297 to accommodate the shaftand/or pitch linkage of adjacent rotors.

As the rotor passes through the hole 293 the rotation of the rotor pullsthe plate about the circular path between the two grooves 289 a, 289 b.The spacing between the hole 293 and the cut-outs 297 on either side ofthe plate is such that when adjacent rotors are at their closest, theadjacent rotor shafts are received in the cut-outs 297 of the centralplate, and the rotor shaft of the central plate is received in thecut-outs 297 of the adjacent plates (as shown in FIG. 15 a). The spacingis also such that when adjacent rotors are at their furthest apart, theadjacent rotors are received in only the cut-outs of the central plate(see FIG. 15 b). In this embodiment of the invention, the ratio betweenthe smallest and greatest separation is 2:1. This is the maximumvariation that can work with a single sliding plate per blade, but forlarger ratios, further plates may be provided.

As demonstrated in FIGS. 15 a and 15 b, there is always a degree ofoverlap between adjacent plates. This maintains a barrier between theinside of the rotor assembly and the outside environment. The hub, driveand pitch mechanisms are therefore protected, to some degree, fromhostile operating environments outside the rotor assembly.

In a further embodiment, that is a variation on the third embodiment,the rotor assembly further comprises a filtered air duct for ducting airflow into the region within the plates. A positive pressure is thereforeestablished in the chamber that further reduces the exposure of movingparts to the outside environment.

According to another embodiment (not shown), the rotor assemblycomprises two perpendicular actuators for moving the drive ring. Atmaximum extension the left/right actuator positions the drive ring asdescribed with reference to the first embodiment. At maximumcontraction, the left/right actuator positions the drive ring over thefar left-hand side of the assembly. A front/back actuator is operable toposition the drive ring forward or aft of the hub centre. Of course, acombination of front/back and left/right positioning on the ring is alsopossible. It will be appreciated that a lateral movement is any movementin any direction within the horizontal plane containing the drive ring.The above-mentioned arrangement is especially useful for varying therotor speed in order to manoeuvre the rotorcraft. For example, byincreasing the speed of rotors on the left-hand side, the net lift forcemoves to the left, causing the helicopter to bank right. By way ofanother example, the drive ring may be moved laterally to the front ofthe rotor assembly, thereby increasing the speed and the lift on therear rotors, causing the helicopter to pitch forward.

Whilst the present invention has been described and illustrated withreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not specifically illustrated herein. For example,in a further embodiment, a rotor assembly comprises a set of rotorsfixedly attached to a rotatable hub and the rotor assembly comprises thepitch control mechanism substantially as described herein with referenceto the first embodiment. In another embodiment of the invention theassembly comprises a plurality of independently controllable motors formoving the rotors around the hub. The angular speed of the rotors aroundthe hub, relative to one another can be varied by independentlycontrolling the speed of the different motors. Where in the foregoingdescription, integers or elements are mentioned which have known,obvious or foreseeable equivalents, then such equivalents are hereinincorporated as if individually set forth. Reference should be made tothe claims for determining the true scope of the present invention,which should be construed so as to encompass any such equivalents. Itwill also be appreciated by the reader that integers or features of theinvention that are described as preferable, advantageous, convenient orthe like are optional and do not limit the scope of the independentclaims.

1. A rotor assembly for a rotorcraft, the assembly comprising aplurality of rotors rotatable about a hub, wherein the assembly isoperable to vary the angular speed of the rotors about the hub, relativeto one another.
 2. An assembly according to claim 1, comprising drivemeans, the drive means being arranged to drive the rotors at a radialdistance away from the hub.
 3. An assembly according to claim 2 whereinthe drive means is operable to drive each of the plurality of rotors ata different radial distance from the hub.
 4. An assembly according toclaim 3, wherein the drive means comprise a rotatably driven drive ring,the rotors being coupled to the circumference of the drive ring.
 5. Anassembly according to claim 4, wherein the drive ring is laterallymoveable relative to the hub, such that the drive ring is operable todrive each of the plurality of rotors at a different radial distancefrom the hub.
 6. An assembly according to claim 5 wherein the assemblyis arranged such that the tangential speed of the rotors at each radialdistance is equal.
 7. An assembly according to claim 5 wherein therotors are slideably moveable through the circumference of the drivering in a direction along the length of the rotor, such that lateralmovement of the drive ring alters the angular spacing of the rotors. 8.An assembly according to claim 2 wherein each rotor is mounted on thehub via a mounting, the drive means being arranged to drive the rotorsat a location radially outward of the mounting.
 9. An assembly accordingto claim 8 wherein the mounting is slideably moveable around thecircumference of the hub. 10.-11. (canceled)
 12. An assembly accordingto claim 1, each rotor comprising a blade having a tip and a root, and ashaft extending from the root of the blade to the hub, wherein theassembly further comprises a pitch controller for varying the pitch ofthe blade, the pitch controller comprising guide means and a linkagemoveable along the guide means, the linkage being connected to the rotorsuch that the pitch of the blade is variable in dependence on theseparation between the rotor and the guide means.
 13. An assemblyaccording to claim 12 wherein the pitch controller further comprises aplurality of independently actuatable actuators arranged to vary theseparation between the rotor and the guide means.
 14. An assemblyaccording to claim 12 wherein the guide means is flexible such that theguide means may be deformed from a planar configuration to a non-planarconfiguration.
 15. An assembly according to claim 12 wherein the linkageis arranged in relation to the guide means such that both tensile andcompressive loads in the linkage are reacted through the pitchcontroller. 16.-17. (canceled)
 18. An assembly according to claim 12wherein the guide means is fixed such that the linkage rotates with therotors relative to the guide means.
 19. (canceled)
 20. An assemblyaccording to claim 12 further comprising a flap controller, the flapcontroller comprising second guide means and a second linkage moveablealong the guide means, the linkage being connected to the rotor suchthat the flap angle of the rotor blade is variable in dependence on theseparation between the rotor and the second guide means. 21.-26.(canceled)
 27. A rotorcraft comprising the rotor assembly according toclaim
 1. 28. A method of moving the centre of lift of a rotorcraft, themethod comprising the steps of rotating a plurality of rotors about ahub such that the angular speed of the rotors about the hub, relative toone another, is varied.
 29. (canceled)
 30. An assembly according toclaim 1, wherein the hub is fixed such that the rotors are rotatableabout, and relative to, the fixed hub.
 31. A rotor assembly for arotorcraft, the assembly comprising a plurality of rotors moveable alonga track around the circumference of a fixed hub such that the rotors arerotatable around the hub, the rotor assembly comprising a drivingmechanism that is operable to vary the angular speed of the rotorsrelative to one another, as the rotors pass around a circumference ofthe fixed hub.